Radio-frequency identification tag with oscillator calibration

ABSTRACT

A method of calibrating an oscillator within a Radio-Frequency Identification (RFID) tag includes storing a plurality of calibration values within a memory structure. Each of the calibration values corresponds to a respective oscillation frequency of the oscillator. A selected calibration value is selected from the plurality of calibration values stored, according to a first selection criterion. The oscillator is then calibrated in accordance with the selected calibration value.

CLAIM OF PRIORITY

This application is Continuation of U.S. application Ser. No. 10/824,073filed Apr. 13, 2004, now U.S. Pat No. 7,120,550, issued on Oct. 10,2006, which is incorporated herein by reference.

TECHNICAL FIELD

An embodiment relates generally to the field of oscillator calibrationand, more specifically, to an apparatus and a method to calibrate anoscillator for a radio-frequency identification (RFID) system.

BACKGROUND

Radio-frequency identification tags (or transponders) require areference frequency for a number of purposes. An RFID reader transmitsRF power to RFID tags. RFID tags modulate reflected RF power to transmitdata back to an RFID reader. The reflected RF is called ‘backscatter,’and the link from the tag back to the reader is typically referred to asthe ‘backscatter link. The backscatter modulation of course requires abackscatter frequency to which the relevant RFID reader is sensitive.Furthermore, backscatter communications may be subject to regulatoryrestrictions, and may need to be compliant with one or more RFIDcommunications specifications or standards. An RFID tag also requires ademodulation frequency so as to enable a demodulator within the RFID tagto demodulate received radio-frequency signals, and decode datacontained therein. RFID tags also need to generate internal clocksignals to clock various functional units that may be included withinthe RFID tag.

With a view to generating the above-identified frequency and clocksignals within an RFID tag, the RFID tag is typically equipped with anoscillator that generates the reference frequency. Three prior artmechanisms for providing such a reference frequency are discussed below.FIG. 1 is a schematic illustration of a first prior art oscillatorarrangement 10 in which an oscillator 12 is coupled to a crystal 14 inorder to provide a precise local reference frequency. Alternatively, theoscillator 12 may be coupled to an L-C tank or electron mobility-basedreference in order to provide the precise local reference frequency. Adisadvantage of such arrangements is that they tend to be bulky, andhigh-power consumers.

A second manner in which it is known to provide a reference frequencywithin an RFID chip is to provide a phase-locked loop (PLL) arrangement,such as that illustrated by the schematic diagram of FIG. 2.Specifically, the phase-locked loop arrangement 16 of FIG. 2 is shown toinclude a phase detector 18 that is coupled to receive a referencefrequency 20 and the oscillator output, compare them, and to provide areference signal 22 to an oscillator 24. The disadvantages of thephase-locked loop arrangement 16 shown in FIG. 2 include the requiredprovision of a reference frequency, a long start-up time, the provisionof extra power for the phase detector 18, as well as the extra chip arearequirements for provision of the phase detector 18. A similar functioncan also be done with a frequency detector, and a frequency-locked loop.

A third prior art arrangement 26 to provide a reference frequency withinan RFID tag is illustrated by the schematic diagram of FIG. 3.Specifically, a trimming arrangement 28 comprising a combination ofresistors, capacitors and inductors (or fuses or resistors that may belaser-cut) provide a reference signal 22 (e.g., a current referencesignal I_(ref)) to an oscillator 30. Among the disadvantages of thisarrangement are that the trimming arrangement may be expensive to build,and the configuration of the trimming arrangement 28 is permanent (i.e.,the oscillator 30 cannot be dynamically calibrated).

FIGS. 4 and 5 are diagrammatic representations of a prior art RFIDsystem 32 including an RFID tag 34 that is interrogated by, and respondsto, an RFID reader 36 utilizing a radio-frequency forward link and abackscatter return link. The RFID tag 34 is shown to provide a signalreceived from the RFID reader 36, via the radio-frequency forward link,to a demodulator 38, which recovers a timing (or clock) signal 40. Therecovered clock signal 40 is utilized to generate a digital calibrationvalue 44, which is stored in a volatile register 42. The volatileregister 42 in turn provides the digital calibration value 44 to adigitally-controlled oscillator (DCO) 46. The digitally-controlledoscillator 46 outputs a demodulator clock signal 48.

FIG. 5 illustrates the oscillator 46 of the RFID tag 34, againcalibrated utilizing a digital calibration value 44 provided to theoscillator 46 from the volatile register 42. The oscillator 46 generatesa modulator clock signal 52 to a modulator 50, the modulator 50utilizing the modulator clock signal 52 to backscatter modulatecommunications transmitted via the backscatter return link to the RFIDreader 36.

In summary, it will be appreciated that, on start-up, the RFID reader 36sends a radio-frequency forward link signal to the RFID tag 34, whichextracts a timing (or clock) signal 40 from the received signal tocalibrate the oscillator 46, this recovered timing signal 40 beingcommunicated to the oscillator 46 via the register 42. Duringbackscatter communications, the calibration is held by the oscillator46, which is in turn utilized to drive the modulator 50.

Accordingly, in the prior art system shown in FIGS. 4 and 5, therecovered timing signal 40 is stored within a volatile register that isutilized to calibrate the oscillator. However, a clock recoveryoperation is required by the demodulator 38 upon each power-up event,which may negatively impact the performance of the RFID tag 34.

U.S. Pat. No. 5,583,819, entitled “Apparatus and Method of Use ofRadiofrequency Identification Tags”, to Bruce B. Roesner and Ronald M.Ames, discloses an RFID tag in which a reference signal is initiallygenerated by comparing an incoming standard signal, and placing it in atemporary or permanent storage within the RFID tag. Signals arrivinglater are then compared to the captured standard, and variations fromthe captured standards are detected to allow for decoding of the data.Specifically, a microprocessor is described as providing a correctionsignal to a memory, the correction signal then being stored within thememory as a correction value for use in subsequent operation of the RFIDtag, or at least until the correction value is updated. The memory isdescribed as possibly being a non-volatile memory to allow calibrationinformation to be permanently stored, so that reconfiguration of theinternal oscillator is not required each time the RFID tag is poweredup.

In the system described by Roesner, the calibration of the oscillator isnonetheless dependent upon an initial extraction or recovery of timingfrom a received radio-frequency signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements and in which:

FIG. 1 is a schematic illustration of a first prior art oscillatorarrangement in which an oscillator is coupled to a crystal in order toprovide a precise local reference frequency.

FIG. 2 is a schematic illustration of a phase-locked loop arrangementthat includes a phase detector coupled to receive a reference frequencyand the oscillator output, to compare them, and to provide a referencesignal to an oscillator.

FIG. 3 is a schematic illustration of a prior art trimming arrangementto provide a reference frequency within an RFID tag.

FIGS. 4 and 5 are diagrammatic representations of a prior art RFIDsystem including an RFID tag that is interrogated by, and responds to,an RFID reader utilizing a radio-frequency forward link and abackscatter return link.

FIG. 6 is a block diagram illustrating multiple operation types that maybe performed by a radio-frequency identification (RFID) integratedcircuit (IC) suitable for use within an RFID tag assembly.

FIGS. 7A, 7B, 8A, and 8B are block diagrams providing high-leveldepictions of systems, in which one or more calibration values may beprovided to an RFID integrated circuit, and written into a non-volatilememory associated with such an RFID integrated circuit.

FIG. 9 is a block diagram illustrating an RFID tag, according to anexemplary embodiment of the present invention, that includes one or moreantennae coupled to an RFID integrated circuit.

FIG. 10 is a diagrammatic representation of an RFID tag, according to afurther exemplary embodiment of the present invention, which againincludes one or more antennae and an RFID integrated circuit.

FIG. 11 is a diagrammatic representative of yet a further exemplaryembodiment of an RFID tag, according to an exemplary embodiment of thepresent invention.

FIG. 12 is a diagrammatic representation of an RFID tag, according toone further exemplary embodiment of the present invention, wherein clockgeneration circuitry of an RFID integrated circuit includes avoltage-controlled oscillator (VCO).

FIG. 13 is a flowchart illustrating a method, according to an exemplaryembodiment of the present invention, to program calibration of anoscillator within a radio-frequency identification (RFID) integratedcircuit for use in a RFID tag.

FIG. 14 is a flowchart illustrating a method, according to a furtherembodiment of the present invention, to program calibration of anoscillator within a radio-frequency identification (RFID) integratedcircuit, using a test device.

FIG. 15 is a flowchart illustrating a method, according to an exemplaryembodiment of the present invention, to calibrate an oscillator within aradio-frequency identification (RFID) circuit that may form part of anRFID tag, and to generate various clock signals within the RFID circuitin accordance with an output of the oscillator.

FIGS. 16-24 are diagrammatic representation providing high-levelrepresentations of various exemplary embodiments of the presentinvention.

FIG. 25 is a schematic diagram illustrating a portion of exemplary clockgeneration circuitry including a core oscillator, and a calibrationmodule.

DETAILED DESCRIPTION

A calibrated oscillator for an RFID system, and methods of manufacturingand operating the same, are described. In the following description, forpurposes of explanation, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be evident, however, to one skilled in the art that the presentinvention may be practiced without these specific details.

FIG. 6 is a block diagram illustrating multiple operation types 60 thatmay be performed by a radio-frequency identification (RFID) integratedcircuit (IC) suitable for use within an RFID tag assembly. In anexemplary embodiment, an RFID tag may be a combination of an RFIDcircuit (e.g., an Integrated Circuit (IC)), and a coupled antenna (orantennae) to facilitate the reception and transmission ofradio-frequency signals, the RFID circuit and the antenna(e) beinglocated on a base material or substrate (e.g., a plastic or papermaterial) to thereby constitute an RFID tag.

As shown in FIG. 6, according to one aspect of the present invention, anRFID integrated circuit may be subject to a programming operation 62, inwhich one or more calibration values are stored within a non-volatilememory (e.g., a floating-gate MOSFET non-volatile memory). The storageof the calibration values may be performed, for example, to facilitatecalibration of an oscillator included within the RFID integrated circuit(in accordance with the one or more calibration values) in advance of aninterrogation operation 64. In one embodiment, each calibration value isa delta value according to which the oscillation of the oscillator,within the RFID integrated circuit, is modified. Various exemplarymethods by which a calibration values may be written to the non-volatilememory, while an RFID integrated circuit is performing a programmingoperation 62, are described below.

FIG. 6 also illustrates that an RFID integrated circuit may perform aninterrogation operation 64, during which the RFID integrated circuitreceives a request from an RFID reader, and then retrieves (orgenerates) reply information, which is encoded in a backscattermodulated radio-frequency signal transmitted from the RFID tag back tothe RFID reader. The backscatter modulation is performed utilizing theone or more calibration values stored within the non-volatile memory.The data included within the backscatter modulated radio-frequencysignal may include, for example, one or more identification codes (e.g.,an Electronic Product Code (EPC)) stored in a memory of the RFID tag. Anumber of exemplary embodiments of methods by which such backscattermodulation may be achieved, and by which various oscillation frequenciesand clock signals may be generated within an RFID integrated circuit,are discussed below.

Dealing first with examples of programming operations 62, FIGS. 7A, 7B,8A, and 8B are block diagrams providing high-level depictions of systems66 and 67, by which one or more calibration values may be provided to anRFID integrated circuit, and written into a non-volatile memoryassociated with such an RFID integrated circuit. Specifically, a testsystem 66 shown in FIG. 7A includes an RFID integrated circuit testdevice 68 that is coupled to an RFID integrated circuit 70 so as toenable the test device 68 to provide a test signal to the RFIDintegrated circuit 70. To this end, the RFID integrated circuit 70includes a suitable interface (not shown) to receive the test signalfrom the test device 68. The test device 68 may be any one of a numberof test devices (e.g., a wafer testing device, a die testing device, oran individual IC testing device) that are commonly used in ICfabrication to test the functionality of integrated circuits. As such,the RFID integrated circuit 70 may be included in a semiconductor waferthat is undergoing testing, and the test device 68 may comprise aprobe-tester.

As shown in FIG. 7A, the test device 68 includes a calibration module 72that is responsible for the inclusion of calibration data within a testsignal supplied to the RFID integrated circuit 70 during testing. Thecalibration module 72 operates to include calibration data (e.g., acalibration command and an update value) within the test signal, thecalibration data causing calibration values 76 to be stored thenon-volatile memory of the RFID integrated circuit 70. For example, thecalibration data may include an update value by which a previouslystored calibration value 76 is to be incremented or decremented so as toproperly calibrate an oscillator 82 included within the RFID integratedcircuit 70. Alternatively, the update valve may itself constitute acalibration value 76 to replace a previously stored calibration value orto be stored as an initial calibration value. The oscillator 82, alsodescribed in further detail below, is utilized in the provision of clocksignals to various components (e.g., a modulator) within the RFIDintegrated circuit 70. The frequency of signals generated by theoscillator 82 may be at least partially determined by the calibrationvalues 76.

In the exemplary embodiment illustrated in FIG. 7A, the test device 68is also shown to include a transmit/receive (TX/RX) interface 73 viawhich the test device 68 communicates test signals to the RFIDintegrated circuit 70, and via which the RFID integrated circuit 70communicates test result data back to the test device 68. As notedabove, the calibration data provided by the calibration module 72 to theRFID integrated circuit 70 may include a calibration command, and anupdate value. The update value may comprise a delta value by which apreviously stored calibration value is to be incremented or decremented.Alternatively, the update value may itself constitute a calibrationvalue to be stored directly into the non-volatile memory 78. In eitherembodiment, the calibration module 72, as a component of the test device68, is responsible for the calculation of one or more update values. Tothis end, the calibration module 72 is additionally configured toreceive test data back from the RFID integrated circuit 70, via thetransmit/receive interface 73, and to determine whether the generationand communication of a further update value is required in order toproperly calibrate the RFID integrated circuit 70. The test datareceived by the calibration module 72 may comprise thebackscatter-modulated output of a modulator included within the RFIDintegrated circuit 70. In this case, the calibration module 72 recoverstiming information from the received backscatter modulated signal todetermine whether the frequency modulation of this signal is correct. Ifnot, the calibration module 72 generates an update value with which tomodify or replace a currently stored calibration value 76. Accordingly,in the exemplary embodiment, described with reference to FIG. 7A, thelogic for the calculation of appropriate calibration values 76 is shownto reside within the test device 68.

FIG. 7B illustrates an alternative embodiment for the test system 66,wherein the calibration module 75, and accordingly the logic for thecalculation and generation of update values, resides in the RFIDintegrated circuit 70 itself, and not within the test device 68. In thisexemplary embodiment, one of the test signals propagated by thetransmit/receive interface 73 of the test device 68 may be a frequencysignal, which is received by the calibration module 75, and utilized torecover timing or clock information. The calibration module 75 is alsoshown to receive, as input, the output of an oscillator 82 of the RFIDintegrated circuit 70. By comparison of the output of the oscillator 82and the recovered timing information, the calibration module 75 maycalculate an appropriate calibration value 76 according to which theoscillator 82 should be calibrated. Having calculated an appropriatecalibration value 76, the calibration module 75 proceeds to write thiscalibration value 76 into the non-volatile memory 78.

The system 67 shown in FIG. 8A includes an RFID reader 84 that includesa calibration module 86 to include calibration data in radio-frequencysignals communicate to an RFID integrated circuit 70 to facilitate thegeneration and/or storage of the calibration values 76 in thenon-volatile memory 78 of an RFID integrated circuit 70.

The system 67 shown in FIG. 8A includes a calibration module 86, forexample similar to the calibration module 72 described above, within theRFID reader 84. Accordingly, in this embodiment, the calibration logicresides largely with the RFID reader 84. FIG. 8B, on the other hand,shows an alternative embodiment of the system 67, wherein a calibrationmodule 87 resides within the RFID integrated circuit 70. As with theembodiments described above with reference to FIGS. 7A and 7B, theembodiments illustrated in FIG. 8A and 8B differ. The RFID reader 84, inthe embodiment illustrated in FIG. 8A, receives a backscatter modulatedsignal from the RFID integrated circuit 70, in response to a programmingsignal, with update values being generated at the RFID reader 84 andthen communicated back to the RFID integrated circuit 70. In theembodiment illustrated in FIG. 8B, on the other hand, the calibrationmodule 87 may, as described above, recover timing information from theprogramming signal, and generate and store the calibration value 76based on the recovered timing information.

From FIGS. 7A, 7B, 8A and 8B, it will be appreciated that calibrationdata, according to various embodiments of the present invention, may beprovided to an RFID integrated circuit 70 by a test device 68 or an RFIDreader 84. Furthermore, the communication of the calibration data is notlimited to communication via a radio-frequency link. In otherembodiments of the present invention, the calibration data may beprovided to the RFID integrated circuit 70 to via a wire link (e.g., viaprobes of a test device 68). The systems 66 and 67 are also merelyexemplary systems by which calibration data may be imparted to, and/orstored within, an RFID integrated circuit 70.

FIG. 9 is a block diagram illustrating an RFID tag 100, according to anexemplary embodiment of the present invention, that includes one or moreantennae 102 coupled to an RFID integrated circuit 106, the antennae 102and integrated circuit 106 being coupled via one or more pads 104, andaccommodated on a common substrate or base material.

Turning specifically to the RFID integrated circuit 106, a front-end ofthe circuit 106 includes a rectifier 108 that operates to extract powerfrom a forward link radio-frequency signal, received via the antenna 102and communicated to the rectifier 108 via one or more pads 104. Therectifier 108 is coupled to provide extracted power to a power regulator114, which in turn provides a regulated voltage (V_(DD)) to variouscomponents of the integrated circuit 106.

The front-end also includes a demodulator 110 that demodulates receivedradio-frequency signals, and extracts received (RX) data there from,which is then communicated to a tag controller 118. The received dataincludes, for example, commands, and associated command data, that areissued from an RFID reader (not shown) to interrogate the RFID tag 100.The commands included within the received data may be commandsconforming to an RFID communications protocol (e.g., EPC Radio-FrequencyIdentification Protocol, as specified by the EPC Global Hardware ActionGroup).

The front-end further includes a modulator 112 that operates to modulatetransmission (TX) data that is supplied to the modulator 112 from thetag controller 118. The transmission data may include, for example, datathat is retrieved from a tag memory 120 by the tag controller 118, andis provided in a reply responsive to commands included within thereceived data. This data may include a programmed identification code(e.g., an EPC). The modulator 112 operates to backscatter modulate thetransmission data, and to provide a backscatter modulated transmissionsignal to the antenna 102, which then transmits a backscatterradio-frequency signal.

A back-end of the RFID integrated circuit 106 includes the tagcontroller 118 and associated tag memory 120. In one exemplaryembodiment, the tag controller 118 may conceptually be regarded as a“core” of the RFID circuit 106. The tag controller 118 includes acommand decoder 122 to decode commands received within the receiveddata, and to control a state occupied by a tag state machine 124,responsive to the commands. Specifically, the tag controller 118 mayoutput specific information, and perform certain actions, depending uponthe state occupied by the tag state machine 124. As such, thetransmission data outputted by the tag state machine may constitute areply to a specific command included within the received data.

The various components of the RFID integrated circuit 106 requirerespective clock signals to synchronize operations, and also properly toprocess information received at and transmitted from the RFID integratedcircuit 106 (e.g., the demodulator 110 and the modulator 112 eachrequire respective clock signals to enable proper demodulation andmodulation.). To this end, the RFID integrated circuit 106 includesclock generation circuitry 127. In exemplary embodiment, the clockgeneration circuitry 127 includes a digitally-controlled oscillator(DCO) 128 that is shown to receive, as a control input, a calibrationvalue 126 stored within the non-volatile tag memory 120. The calibrationvalue 126 causes of the frequency of the oscillator 128 to be calibratedto a desired frequency (e.g., a backscatter modulation frequency). Incertain embodiments, a register (not shown) may be interposed betweenthe non-volatile tag memory 120 and the digitally-controlled oscillator128. The oscillator 128 in turn outputs a frequency signal 130 (e.g., asquare wave signal), the frequency signal 130 providing input to acounter module 132. The counter module 132 may include one or morecounters that utilize the frequency signal 130 to generate one or moreclock signals. For example, the counter module 132 may utilize thefrequency signal 130 to generate a modulator clock signal 136 that isprovided to the modulator 112, so as to enable the modulator 112 tobackscatter modulate the transmission data. The counter module 132 isalso shown to provide various clock signals to other components of theRFID integrated circuit 106. It will be appreciated that these variousclock signals may in fact be the same clock signal, or may be differentclock signals, depending upon the requirements of the variouscomponents. Further, the counter module 132 may, in one embodiment, formpart of the tag controller 118.

It will also be noted that the tag state machine 124 provides a commandsignal to the counter module 132, in the exemplary form of amultiplication signal 134, which controls the manner in which thecounter module 132 generates respective clock signals. For example, acounter within the counter module 132 that is utilized to generate themodulator clock signal 136 may be controlled by the multiplicationsignal 134 to control the frequency of the modulator clock signal 136.In this embodiment, the frequency with which the modulator 112 modulatesa backscatter radio-frequency signal is thus controlled at leastpartially by the multiplication signal 134. As such, the modulation ofthe backscatter radio-frequency signal may be performed in accordancewith both the oscillation frequency signal 130, that is determined bythe calibration value 126, as well as the command signal, in theexemplary form of the multiplication signal 134, that provide input tothe counter module 132. Of course, clock signals other than themodulator clock signal 136 may similarly be generated utilizing thefrequency signal 130 and the multiplication signal 134.

The exemplary RFID integrated circuit 106 illustrated in FIG. 9 presentsa number of advantages. As the oscillation of the oscillator 128 iscalibrated utilizing the calibration value 126, which is pre-storedwithin the non-volatile tag memory 120 and is not recovered from aradio-frequency signal received on the radio-frequency forward link,operational speed of the RFID integrated circuit 106 may be improved.For example because the oscillator 128 does not require calibrationrelative to a recovered clock signal on every power-on, performanceadvantages may be achieved. Further, by allowing the modulation ofvarious clock signals within the RFID integrated circuit 106 to bemodified responsive to commands received at the tag 100, an RFID readeris provided with control over backscatter radio-frequency signals thatare issued in response to interrogation signals.

FIG. 10 is a diagrammatic representation of an RFID tag 140, accordingto a further exemplary embodiment of the present invention, which againincludes one or more antennae 102 and an RFID integrated circuit 141.The integrated circuit 141 differs from the exemplary embodiment in FIG.9 in that a dual-oscillator architecture is provided. Specifically, theintegrated circuit 141 includes (1) a modulator/core oscillator 142 thatis utilized to generate a modulator clock signal 144 and a core clocksignal 148, and (2) a demodulator oscillator 146 that is utilized togenerate a demodulator clock 149. The oscillators 142 and 146 aredistinguished in that the modulator/core oscillator 142 may becalibrated utilizing one or more calibration value 126 stored within thenon-volatile tag memory 120, whereas the demodulator oscillator 146 isdriven by timing recovered from a received radio-frequency signal.

The dual-oscillator architecture provides the advantage that the needfor over-sampling of a received radio-frequency signal may be reducedrelative to the over-sampling requirements of the architecture describedabove with reference to FIG. 9. Nonetheless, the advantages provided bycalibrating the modulator/core oscillator 142, utilizing a calibrationvalue 126 stored within a non-volatile memory, as described above withreference to FIG. 9, remain.

FIG. 11 is a diagrammatic representative of yet a further exemplaryembodiment of an RFID tag 150. Again, the RFID tag 150 is comprised ofan antenna 102 coupled to an RFID integrated circuit 152. The RFIDintegrated circuit 152 is shown to include a non-volatile tag memory 154in which are stored multiple calibration values 156, 158. Clockgeneration circuitry 160 includes a selection mechanism, in theexemplary form of a multiplexer (MUX) 162, which is operable by the tagstate machine 124 (in turn responsive to a decoded command) to selectone of the multiple calibration values 156, 158 stored within thenon-volatile tag memory 154. The selected calibration value is thenutilized to drive a digitally-controlled oscillator (DCO) 166, which inturn generates a modulator clock signal 144. For the sake of simplicity,the clock generation circuitry 160 is only shown to generate a modulatorclock signal 144. It will nonetheless be appreciated that the clockgeneration circuitry 160 may be utilized to produce clock signals forany of the components of the RFID integrated circuit 152.

The architecture illustrated in FIG. 11 is advantageous in that anoscillator can accordingly be driven by any one of multiple calibrationvalues stored within a non-volatile tag memory 154, the choice ofcalibration values being controlled by the tag controller 118.

As described above, the selection performed by the tag controller 118may be performed responsive to a command sent, for example, by an RFIDreader and included in the received data extracted by the demodulator110. Consider the situation in which an RFID reader (not shown) requiresthe RFID tag 150 to backscatter at one of a number of possiblebackscatter frequencies. In this embodiment, a number of backscattervalues, corresponding to a number of possible backscatter frequencies,may be stored in the tag memory 154. A command may be then communicatedfrom the RFID reader to the RFID tag 150, instructing a specificbackscatter frequency. Responsive to this command, the tag state machine124 may be placed in a state in which a calibration value, to calibratethe oscillator 166 to generate an appropriate modulator clock signal144, may be selected for input, via the MUX 162, to thedigitally-controlled oscillator 166. In this manner, the output of thetag state machine 124 can be utilized to control the frequency of amodulator clock signal provided to a modulator 112 of an RFID integratedcircuit. However, in the embodiment illustrated in FIG. 11, as opposedto generating a multiplication signal 134, the tag state machine 124outputs a MUX selection signal 164. In other embodiments of the presentinvention, the selection of an appropriate calibration value may beperformed by the tag controller 118 responsive to other inputs orconditions, such as a mode of operation of the RFID tag 150, a sensedtemperature of a component of the RFID tag 150 or of a particularenvironmental (or ambient) condition, a voltage within the RFID tag 150,etc. Information regarding such other inputs or conditions may beprovided to the tag controller 118 via commands received from an RFIDreader, or via sensors that are coupled to the RFID tag 150. In furtherexemplary embodiments, a frequency of an RFID chip may be changed inresponse to process variations as measured by threshold voltage relativeto an on-chip voltage reference, or may be changed in response to ameasure of noise an interference seen by the demodulator.

The storage of multiple calibration values 156 and 158, and the abilityto dynamically select a calibration value to drive an oscillator, isadvantageous in that this allows the clock signals within the RFIDintegrated circuit 152 to be dynamically varied in response to receivedcommands, or monitored internal or external conditions. For example, inorder to render the RFID tag 150 operable in a number of differentregulatory environments, the frequency may need to be adjusted by 10%,for example, to fit within regulatory constraints. In this case, thereader sends a command to switch to an appropriate frequency, responsiveto which the RFID tag 150 would switch to the correct frequency. Inanother embodiment, a sensor (or other component, e.g., the demodulator)may provide an indication that received power is low and the clock maythen be slowed automatically to save power, at the expense of a reducedset of recognized commands.

FIG. 12 is a diagrammatic representation of an RFID tag 170, accordingto one further exemplary embodiment of the present invention. Thearchitecture of the RFID tag 170 illustrated in FIG. 12 differs fromthat illustrated in FIG. 11 in that the clock generation circuitry 174of the RFID integrated circuit 172 includes a voltage-controlledoscillator (VCO) 182, as opposed to the digitally-controlled oscillator166 of the RFID integrated circuit 152. Accordingly, the clockgeneration circuitry 174 is shown to include a register 178 to store aselected calibration value outputted from the MUX 176, adigital-to-analog converter (DAC) 180 to convert the selectedcalibration value stored in the register 178 to a voltage signal, thevoltage control oscillator 182, and a counter 184.

FIG. 13 is a flowchart illustrating a method 200, according to anexemplary embodiment of the present invention, to program calibration ofan oscillator within a radio-frequency identification (RFID) integratedcircuit for use in a RFID tag. The programming performed in the method200 is performed by an RFID reader device, which communicates with theRFID tag utilizing a radio-frequency forward link. The method 200 may beperformed within the context of a system 67 such as that shown in FIG.8A.

The method 200 commences at block 202 with the RFID reader transmittinga calibration mode command to the RFID tag, in order to place the taginto a programming mode (e.g., the programming mode 64 discussed abovewith reference to FIG. 6). Responsive to receipt of the programming modecommand, at block 204, the RFID integrated circuit enters a programmingmode in which the RFID reader is provided with command access to anon-volatile memory that forms part of the RFID integrated circuit, oralternatively is a distinct non-volatile memory to which the RFIDintegrated circuit has access.

At block 206, the RFID reader then proceeds to transmit a calibrationcommand to the RFID tag, the calibration command in one exemplaryembodiment instructing the RFID tag to write an update value to anon-volatile tag memory of the RFID tag. In the exemplary embodiment,the calibration command takes the form of a “write” command in thefollowing format:

[Preamble: 6-bit], [Command: 8-bit], [Memory Address: 2-bit], [Data:16-bit], [CRC: 8-bit]

The 8-bit command is recognized by a command decoder 122 of a tagcontroller 118 as specifying a write command, with the data (e.g., theupdate value) being included within the 12-bit data portion of the writecommand.

Returning to the flowchart illustrated in FIG. 13, at block 208, theRFID integrated circuit, having received a radio-frequency signal fromthe RFID reader in which the command is modulated, demodulates thereceived radio-frequency signal utilizing the demodulator 110, andcommunicates the command data to the command decoder 122 of the tagcontroller 118. The command decoder 122 then provides the appropriatecommand information to the tag state machine 124 which then proceeds towrite the included update value into the non-volatile memory 120associated with the RFID integrated circuit.

As was noted above, the update value that is communicated as part of thecommand data at block 206, and that is received by the RFID integratedcircuit, may itself constitute a calibration value, which is thenwritten to the non-volatile memory 120. In an alternative embodiment,the update value may be a value by which a previously calculated andstored calibration value 126 is to be modified. In this case, thecommand associated with the update value may further instruct anincrement or a decrement operation with respect to a stored calibrationvalue 126, utilizing the update value. In this embodiment, theoperations performed at block 208 accordingly include the performance ofan appropriate increment or decrement operation to thereby generate anew calibration value 126 to be written into the non-volatile memory120.

The method 200 then proceeds to decision block 210, where the RFIDreader determines whether any further calibration values are to bewritten into the non-volatile memory of the target RFID tag. Forexample, as noted above with reference to FIGS. 11 and 12, multiplecalibration values 156, 158 may be stored within the non-volatile memoryof an RFID tag. In the event that it is determined at decision block 210that further calibration values are in fact to be programmed, the method200 loops back to block 206.

On the other hand, if no further calibration values are to beprogrammed, the method 200 proceeds to block 212, and the RFID readertransmits an exit programming mode command to the RFID tag, responsiveto which the RFID integrated circuit exits programming mode at block214. The exit programming mode command may be a “lock” command thatoperates to prevent subsequent write operations to the non-volatilememory of the RFID tag. The method 200 then terminates at block 216.

While the method 200 is described above as having the RFID readertransmit a programming mode command and an exit programming mode commandto the RFID integrated circuit to render the RFID integrated circuitprogrammable and non-programmable with respect to update values, it willbe appreciated that, in other embodiments of the present invention, theRFID integrated circuit could automatically enter a programming modeupon receiving a calibration command, such as that discussed withreference to block 206.

It is worth noting that the programming of the calibration values intothe non-volatile memory of the RFID tag, as discussed above withreference to FIG. 13, is not dependent upon the frequency of theradio-frequency signal transmitted by the RFID reader. In other words,the calibration value that is written into the non-volatile memory isnot derived from a frequency of the forward link radio-frequency signalitself, but is rather communicated as, or derived from, a specific valueassociated with a command communicated from the RFID reader to the RFIDtag.

FIG. 14 is a flowchart illustrating a method 220, according to a furtherembodiment of the present invention, to program calibration of anoscillator within a radio-frequency identification (RFID) integratedcircuit. The programming performed in the method 220 is performed by atest device 68, for example within the context of a system 66 asdescribed above with reference to FIG. 7B.

The method 220 commences at block 222 with the initiating and testing ofan RFID integrated circuit. The testing of the RFID integrated circuitmay be as part of the testing of an entire wafer on which the RFIDintegrated circuit is included, a die including the RFID integratedcircuit, of the RFID integrated circuit once rendered as a distinctchip, or as part of testing the assembled RFID tag.

At block 224, the RFID integrated circuit optionally enters aprogramming mode. For example, a test device 168 may issue a programmingmode command to the RFID integrated circuit 70 to cause the RFIDintegrated circuit to transition into the programming mode. At block226, the test device 68 then provides a test signal to the RFIDintegrated circuit 70. In one embodiment of the present invention, thetest signal has predetermined reference frequency that the RFIDintegrated circuit 70 utilizes to record a calibration value 76 within anon-volatile memory associated therewith. In alternative embodiments,the test signal may include a command, and an associated update value,for a specification of a calibration value 76. Further, in oneembodiment, the test signal may be a DC power line test signal that isapplied to the RFID integrated circuit. At block 228, the RFIDintegrated circuit recovers the reference frequency from the providedtest signal. Specifically, the calibration module 75 of the RFIDintegrated circuit 70 may operate, as described above, with reference toFIG. 7B to extract the reference frequency from the received testsignal, and to compare the extracted reference frequency to a currentfrequency of the oscillator 82. Based on this comparison, thecalibration module 75 then calculates a calibration value, appropriateto calibrate the oscillator 82 to the extracted reference frequency.

At block 230, the RFID integrated circuit then stores the calibrationvalue, corresponding to the reference frequency, within the non-volatilememory. Specifically, the calibration module 75 may proceed to write thecalibration value 76 into the non-volatile memory, as illustrated inFIG. 7B. The operations performed at blocks 228-230 may be iterativelyperformed in order to determined the proper calibration value to bestored at block 230.

At decision block 234, a determination may be made whether any furthercalibration values (e.g., corresponding to alternative backscattermodulation frequencies) need to be programmed. This determination may bemade at the test device 68 or may alternatively be made at thecalibration module 75, responsive to which the calibration module 75 mayprovide an appropriate signal back to the test device 68. In the eventthat further calibration values are to be programmed, the method 220then loops back to block 226. On the other hand, should no furthercalibration values need to be programmed, the method 220 proceeds toblock 236, where the RFID integrated circuit 70 exits programming mode.The method then terminates at block 238.

FIG. 15 is a flowchart illustrating a method 240, according to anexemplary embodiment of the present invention, to calibrate anoscillator within a radio-frequency identification (RFID) circuit thatmay form part of an RFID tag, and to generate various clock signalswithin the RFID circuit accordance with an output of the oscillator.

The method 240 commences at block 242 with the receipt, at an RFID tag,of a radio-frequency interrogation signal from an RFID reader. Theflowchart of FIG. 15 depicts two high-level operations as performedwithin the RFID integrated circuit of the RFID tag. Specifically, asdesignated generally at 243, a recovered clock signal may optionally begenerated within the RFID tag based on the received radio-frequencyinterrogation signal. Separately, and possibly concurrently, asdesignated generally at 251, one or more programmed clock signals mayalso be generated within the RFID integrated circuit of the RFID tag.While the generation of the recovered clock signal at 243 may bedependent upon the reception of the radio-frequency interrogationsignal, the generation of the programmed clock signals at 251 is notnecessarily dependent upon reception of an interrogation signal, as willbe more fully appreciated from the below reading. Specifically, thecalibration of an oscillator within an RFID circuit from a stored valuedoes not presuppose the reception of an interrogation signal.

Turning first to the generation of the recovered clock signal, at block244, timing information is recovered from the received radio-frequencyinterrogation signal. Referring, for example, to the exemplary RFID tagillustrated in FIG. 10, the received interrogation signal is received atthe demodulator 110 of the RFID integrated circuit 141. The demodulator110 includes clock recovery circuitry (not shown) that then proceeds torecover the relevant timing information from the received interrogationsignal. At block 246, the recovered timing information is written fromthe demodulator 110 to a volatile memory 109, associated with thedigitally-controlled demodulator oscillator 146. At block 248, arecovered clock signal is generated utilizing the recovered timinginformation, as stored in the volatile memory 109. Specifically, in oneexemplary embodiment, the timing information stored within volatilememory 109 provides digital input to the digitally-controlleddemodulator oscillator 146, which then outputs a recovered clock signalin the exemplary form of the demodulator clock signal 149.

At block 250, the recovered clock signal is provided to at least onecomponent of the RFID integrated circuit 141. For example, thedemodulator clock signal 149 is provided to the demodulator 110.

Turning now to the generation of a programmed clock signal, which may ormay not occur in parallel with the generation of the recovered clocksignal, at block 252 stored timing information, in the exemplary form ofa calibration value 126, is retrieved from a non-volatile memory (e.g.,the tag memory 120). The stored timing information may have been writteninto the non-volatile memory utilizing any one of the methods describedabove. Where the non-volatile memory stores multiple calibration values,the retrieval of the stored timing information at block 252 may includeselection of a selected calibration value according to one or moreselection criterion, discussed above with reference to FIG. 11.

At block 254, a programmed clock signal is generated utilizing thestored timing information. Again referring to the exemplary embodimentillustrated in FIG. 10, stored timing information in the exemplary formof a calibration value 126 may be provided to a digitally-controlledmodulator/core oscillator 142, which in turn outputs a frequency signalto a counter module 132. The counter module 132 then outputs aprogrammed clock signal in the exemplary signal in the exemplary form ofa modulator clock signal 136.

In the alternative embodiment of the present invention described withreference to FIG. 12, the generation of the programmed clock signal maybe performed utilizing a voltage-controlled oscillator (VCO).

At block 256, the programmed clock signal is provided to at least onecomponent of the RFID integrated circuit. Referring again to theexemplary embodiment illustrated in FIG. 10, the counter module 132 may,for example, provide the modulator clock signal 144 to a modulator 112,as well as provide a core clock signal 148 to at least the tagcontroller 118 of the RFID integrated circuit 141. The method 240 thenterminates at block 258.

In summary, it will be noted that the exemplary method 240 mayoptionally include the generation of both a recovered clock signal and aprogrammed clock signal within a common RFID integrated circuit. To thisend, the relevant RFID integrated circuit may employ the dual-oscillator(or multi-oscillator) architecture discussed above with reference toFIG. 10.

FIGS. 16-24 are diagrammatic representations of various exemplaryembodiments. In FIGS. 16-24, for the purposes of clarity, only selectedcomponents and signals have been illustrated.

Turning first to FIG. 16, an RFID tag 260 is shown to be interrogatedby, and to respond to, an RFID reader 262. The RFID tag 260 receives aforward-link radio-frequency signal, which is communicated to ademodulator 264. The RFID tag 260 further includes adigitally-controlled oscillator (DCO) 266, which is calibrated using acalibration value stored within a non-volatile memory 268, and generatesa demodulator clock signal 270. The calibration value stored within thenon-volatile memory 268 may be written into the memory 268 utilizing anyone of the methods discussed above. Accordingly, a non-volatile memory(NVM) calibrated-oscillator is utilized to generate the demodulatorclock signal 270 to clock the demodulator 264 utilizing timinginformation recovered from the forward-link radio-frequency signal. Inone embodiment, in order to ensure a required accuracy in thedemodulation of the forward-link radio-frequency signal, the demodulatorclock signal 270 may be programmed so that the demodulator 264 oversamples the received forward-link radio-frequency signal. Thearrangement illustrated in FIG. 16 is advantageous in that no trainingsequence is required to calibrate the oscillator 266 based on recoveredtiming information from the forward-link radio-frequency signal.

FIG. 17 is a diagrammatic representation of an RFID tag 271, in whichthe NVM-calibrated oscillator 266 is utilized to drive a modulator clocksignal 272, which is in turn utilized to clock a modulator 274 of theRFID tag 271. Accordingly, it will be appreciated that a frequency of abackscatter-modulated radio-frequency signal 276, transmitted from theRFID tag 271 to the RFID reader 262, is related to the frequency of themodulator clock signal 272. It should furthermore be noted that themodulator clock signal 272 is not necessarily related to the demodulatorclock signal 270. For example, the demodulator and modulator clocksignals 270 and 272 may be driven by different calibration values storedwithin the non-volatile memory 268. Further, in a dual-oscillatorarchitecture, separate oscillators may be provided to generate each ofthe demodulator and modulator clock signals 270 and 272.

FIG. 18 is a block diagram illustrating an exemplary RFID tag 280, inwhich an NVM-calibrated clock signal 282 is provided to a digital core284 of the RFID tag 280. Again, the system clock signal 282 need notnecessarily be related to the demodulator and modulator clock signals270 and 272 discussed above. For example, independent calibration valuesmay be stored within the non-volatile memory 268 to generate each of theclock signals 270, 272 and 282. Further, independent oscillators 266 maybe provided to generate each of these clock signals. Of course, incertain embodiments, each of the clock signals 270, 272, and 282 may, infact, be driven by a common calibration value, stored within a commonnon-volatile memory, and provided to a common oscillator 266.

FIG. 19 is a diagrammatic representation of an RFID tag 290, accordingto one exemplary embodiment of the present invention, with an oscillator266 being driven by any one of a multiple calibration values storedwithin a non-volatile memory structure. Specifically, the provision ofone of the calibration values 292 and 294 to the oscillator 266 is shownto be controlled by an on-chip digital controller, in the exemplary formof the digital core 284. The high-level architecture depiction shown inFIG. 19 could, it will be appreciated, be implemented in the mannerdiscussed above with reference to FIG. 11, wherein calibration values156 and 158 are stored within a non-volatile memory 154, and wherein thedigital core 284 includes the tag controller 118, which in turn includesthe tag state machine 124 that outputs a selection signal 164 to selectbetween one of multiple calibration values. As also noted above, theselection of the appropriate calibration value 292 or 294 may bedependent upon any number of factors, including a mode of operation ofthe digital core 284, commands received at the RFID tag 290 from an RFIDreader 262, tag temperature, tag voltage, etc.

FIG. 20 is a diagrammatic representation of an RFID tag 300 according tofurther embodiment of the present invention, wherein an oscillator 266is driven by a calibration value stored within a non-volatile memory302, or alternatively by a calibration value stored within a volatilememory (e.g., a register 304). The non-volatile memory 302 may itselfstore multiple calibration values between which a selection may also bemade. The choice between a calibration value stored within thenon-volatile memory 302 and the volatile register 304 is, as with theembodiment described below with reference to FIG. 19, controlled by thedigital core 284. Specifically, a digital state (e.g., the state of atag state machine 124) may determine the selection performed by thedigital core 284. For example, when the RFID tag 300 is receiving aforward-link radio-frequency signal that requires demodulation, thedigital core 284 may place the RFID tag 300 in a demodulation state, andaccordingly select a value within the volatile register 304 to drive theoscillator 266, and to output an appropriate demodulation clock to ademodulator (not shown). Alternatively, when the RFID tag 300 istransmitting a backscatter modulated radio-frequency signal as a replyto an RFID reader, the digital core 284 may place the RFID tag 300 in amodulation state, and accordingly select a calibration value within thenon-volatile memory 302 to drive the oscillator 266. The oscillator 266will then accordingly output an appropriate modulation clock signal to amodulator (not shown).

It should furthermore be noted that one or more of calibration valuesstored within the non-volatile memory 302 may be programmaticallywritten and stored within the memory 302, whereas a value stored withinthe volatile register 304 may represent recovered timing information,recovered from a forward link radio-frequency signal received at theRFID tag 300.

FIG. 21 is a diagrammatic representation of an RFID tag 310, accordingto one embodiment of the present invention, where a singleNVM-calibrated oscillator 266 is utilized to drive modulator,demodulator, and system clock signals 314, 318, and 319. This embodimentis in contrast to a further exemplary embodiment of an RFID tag 330,illustrated in FIG. 22, which employs a multi-oscillator architecture.Specifically, in the embodiment illustrated in FIG. 22, a firstoscillator 322 is dedicated to the generation of a demodulation clock336, and is driven by timing information recovered from a forward-linkradio-frequency signal, and represented by a calibration value storedwithin a volatile register 334 that provides input to the oscillator332. A second oscillator 340 is responsible for the generation of amodulator clock signal 348 and a system clock signal 344. The secondoscillator 340 is calibrated utilizing a calibration value stored withina non-volatile memory 342, which provides input to the second oscillator340.

FIGS. 23 and 24 are diagrammatic representations of an RFID tag 360,according to an even further exemplary embodiment of the presentinvention, with a single oscillator which is selectively calibratedutilizing values stored within a non-volatile memory 366 and valuesstored within a volatile memory, in an exemplary form of a volatileregister 364. The non-volatile memory 366 and the volatile register 364provide an example of a tag memory structure. As has been discussed indetail above, the selective provision of a calibration value from eitherthe non-volatile memory 366 or the volatile register 364 is controlledby a digital core 372, and may be based on a state occupied by the RFIDtag. FIG. 23 illustrates that, during data recovery, the oscillator 362is driven by a calibration value stored within the volatile register364, to generate a demodulator clock signal 368. The calibration valuestored within the volatile register 364 is furthermore shown to reflectrecovered timing information, as generated an outputted by a demodulator370. Accordingly, during data recovery, the demodulator clock signal 368may be set according to timing information recovered from a forward linkradio-frequency signal received at the RFID tag 360.

FIG. 24, on the other hand, illustrates that during modulation, theoscillator 362 may be driven by a calibration value stored within thenon-volatile memory 366. As discussed above, the calibration valuestored within the non-volatile memory 366 may be programmed (e.g.,during a programming event or mode). Accordingly, the oscillator 362 isutilized to drive a programmed clock signal, in the exemplary form ofthe modulator clock signal 374, that is provided to the modulator 376.Accordingly, the modulator 376 backscatter modulates a transmittedradio-frequency signal in accordance with the received modulator clocksignal 374. The calibration modules discussed above may employ any oneof a number of calibration algorithms in order to determine one or morecalibration values 76 to be stored within the non-volatile memory of anRFID tag. In one exemplary embodiment, a calibration module may employthe so-called Successive Approximation Algorithm (SAA) that assumes,without loss of generality, that higher settings give a higherfrequency. Accordingly, the algorithm typically starts with themost-significant bit (MSB), recognizing the MSB as a test bit. The testbit is set to one, and all lower bits are set to zero. The inherentfrequency of an oscillator 82 to be calculated is compared to anexternal reference frequency. As noted above, this reference frequencymay be provided via a radio-frequency (or other air or a wired link(e.g., through probing at test time)). If the observed frequency of theoscillator 82 is too high, the test bit is set to zero, and the nextmost significant bit is selected as the test bit, and set to 1. Theabove process is repeated until the observed frequency of the oscillator82 corresponds to the provided external reference frequency.

In a further embodiment, a calibration module may employ a feedbackalgorithm to generate one or more calibration values to be written intothe non-volatile memory 78 of an RFID integrated circuit 70.Specifically, such an algorithm sets the calibrated output frequency ofan oscillator 82 at a mid-range, and then counts how many external clockcycles (E) pass in a fixed number of internal clock cycles (I). Thealgorithm adjusts the calibrated outward frequency of the oscillator 82proportionately to (E-I) until this value is sufficiently small.

FIG. 25 is a schematic diagram illustrating a portion of exemplary clockgeneration circuitry 380 including a core oscillator 382, andcalibration module 384. Within the calibration module 384, M1 mirrors areference current (I_(ref)) to M2-M5. Calibration is applied to thegates of M6-M9 to control the current supplied to the core oscillator382.

It should also be noted that embodiments of the present invention may beimplemented and not only as a physical circuit or module (e.g., on asemiconductor chip) but, also within a machine-readable media. Forexample, the circuits and designs described above may be stored upon, orembedded within, a machine-readable media associated with a design toolused for designing semiconductor devices. Examples include a netlistformatted in the VHSIC Hardware Description Language (VHDL), the Veriloglanguage, or the SPICE language. Some netlist examples include abehavioral level netlist, a register transfer level, (RTL) netlist, agate level netlist, and a transistor level netlist. Machine-readablemedia include media having layout information, such as a GDS-II file.Furthermore, netlist files and other machine-readable media forsemiconductor chip design may be used in a simulation environment toperform any one or more methods described above. Thus it is also to beunderstood that embodiments of the present invention may be used, or tosupport, a software program executing on some processing core (e.g., aCPU of a computer system), or otherwise implemented or realized within amachine-readable medium. A machine-readable storage medium may includeany mechanism for storing information in a form readable by a machine(e.g., a computer). For example, a machine readable storage medium maycomprise a read-only memory (ROM), a random access memory (RAM),magnetic disc storage media, optical storage media, and/or flash memorydevices.

Thus, a calibrated oscillator for an RFID system, and methods ofmanufacturing and operating the same, has been described. Although thepresent invention has been described with reference to specificexemplary embodiments, it will be evident that various modifications andchanges may be made to these embodiments without departing from thebroader spirit and scope of the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative ratherthan a restrictive sense.

1. A Radio-Frequency Identification (RFID) tag comprising: an antenna;and an integrated circuit coupled to the antenna, comprising: anoscillator; and a tag controller to enter a programming operationdistinct from an interrogation operation in response to receiving acalibration command from an RFID reader and to select a selectedcalibration value, from a plurality of calibration values stored withina memory structure, according to a first selection criterion, each ofthe calibration values corresponding to a respective oscillationfrequency of the oscillator, and the oscillator being operationallycalibrated utilizing the selected calibration value for subsequent usein the interrogation operation.
 2. The RFID tag of claim 1, wherein thetag controller is to receive the calibration command, and an associatedupdate value, and to store at least one of the plurality of calibrationvalues within the memory structure responsive to the at least onecalibration command.
 3. The RFID tag of claim 2, wherein the tagcontroller is to generate at least a first calibration value of theplurality of calibration values within the RFID tag, and wherein theupdate value is a modification value by which a second calibrationvalue, previously generated within the RFID tag, is modified by the tagcontroller to generate the first calibration value.
 4. The RFID tag ofclaim 3, wherein the tag controller is to at least one of increment ordecrement the second calibration value by the modification value tothereby generate the first calibration value.
 5. The RFID tag of claim2, wherein the update value is equal to a first calibration value of theat least one of the plurality of calibration values, the update valuehaving been generated in a calibration device external to the RFID tag,and wherein the tag controller is to store the update value as the firstcalibration value within the memory structure.
 6. The RFID tag of claim1, wherein the first selection criterion includes any one of a group ofselection criterion including a mode of operation of the RFID tag, aselection command received at the RFID tag, an ambient conditionapplicable to the RFID tag, and an internal voltage applicable to theRFID tag.
 7. The RFID tag of claim 1, wherein the tag controller is todetermine at least one of the plurality of calibration values based on aradio-frequency signal received at the RFID tag from the RFID reader,and to store the at least one calibration value in the memory structure.8. The RFID tag of claim 1, wherein the memory structure includes anon-volatile memory and a volatile memory, at least one of the pluralityof calibration values being stored within the non-volatile memory, andat least a further one of the plurality of calibration values beingstored within the volatile memory.
 9. The RFID tag of claim 8, whereinthe at least one calibration value stored in the non-volatile memory isnot related to a frequency of a radio-frequency signal received at theRFID tag from the RFID reader, the oscillator utilizing the at least onecalibration value stored in the non-volatile memory to generate amodulator clock signal to be supplied to a modulator of the RFID tag.10. The RFID tag of claim 8, wherein the at least one furthercalibration value stored in the volatile memory is derived from aradio-frequency signal received at the RFID tag from the RFID reader,the oscillator utilizing the at least one further calibration value togenerate a demodulation clock signal to be supplied to a demodulator ofthe RFID tag.
 11. A method for calibrating an oscillator within aradio-frequency identification (RFID) tag, the method comprising:entering a programming operation distinct from an interrogationoperation in response to receiving a calibration command from an RFIDreader; storing at least one of a plurality of calibration values withina non-volatile memory and at least one other of the plurality ofcalibration values in a volatile memory of a memory structure associatedwith the RFID tag, each of the calibration values corresponding to arespective oscillation frequency of the oscillator; selecting a selectedcalibration value, from the plurality of calibration values storedwithin the memory structure, according to a first selection criterion;and calibrating the oscillator in accordance with the selectedcalibration value for subsequent use in the interrogation operation. 12.The method of claim 11, wherein the storing of the plurality ofcalibration values within the memory structure includes receiving thecalibration command, and an associated update value, at the RFID tag.13. The method of claim 11, including generating at least a firstcalibration value of the plurality of calibration values within the RFIDtag, the update value being a modification value by which a secondcalibration value, previously generated within the RFID tag, is modifiedto generate the first calibration value.
 14. The method of claim 13,wherein the generating of the first calibration value includes at leastone of incrementing or decrementing the second calibration value by themodification value to thereby generate the first calibration value. 15.The method of claim 12, wherein the update value is equal to a firstcalibration value of the plurality of calibration values, the updatevalue having been generated in a calibration device external to the RFIDtag.
 16. The method of claim 11, wherein the first selection criterionincludes any one of a group of selection criterion including a mode ofoperation of the RFID tag, a selection command received at the RFID tag,an ambient condition applicable to the RFID tag, and an internal voltageapplicable to the RFID tag.
 17. The method of claim 11, wherein thestoring of the plurality of calibration values within the memorystructure includes determining at least one of the plurality ofcalibration values based on a radio-frequency signal received at theRFID tag from the RFID reader, and storing the at least one calibrationvalue in the memory structure.
 18. The method of claim 11, wherein theat least one calibration value stored in the non-volatile memory is notrelated to a frequency of a radio-frequency signal received at the RFIDtag from the RFID reader, the method including utilizing the at leastone calibration value stored in the non-volatile memory to generate amodulator clock signal to be supplied to a modulator of the RFID tag.19. A radio-frequency identification (RFID) tag comprising: antennameans for transmitting and receiving wireless communications from anRFID reader; and integrated circuit means for performing tag operations,comprising: first means for entering a programming operation distinctfrom an interrogation operation in response to receiving a calibrationcommand from an RFID reader and selecting a selected calibration value,from a plurality of calibration values stored within a memory structureassociated with the RFID tag, according to a first selection criterion;and second means for generating a frequency signal in accordance withthe selected calibration value for subsequent use in the interrogationoperation.
 20. The RFID tag of claim 19, wherein the first means is forreceiving at least one calibration command, and an associated updatevalue, and for storing at least one of the plurality of calibrationvalues within the memory structure responsive to the at least onecalibration command.
 21. The RFID tag of claim 19, wherein the firstmeans is for generating at least a first calibration value of theplurality of calibration values within the RFID tag, and wherein theupdate value is a modification value by which a second calibrationvalue, previously generated within the RFID tag, is modified by thefirst means to generate the first calibration value.
 22. The RFID tag ofclaim 21, wherein the first means is for at least one of incrementing ordecrementing the second calibration value by the modification value tothereby generate the first calibration value.
 23. The RFID tag of claim20, wherein the update value is equal to a first calibration value ofthe plurality of calibration values, the update value having beengenerated in a calibration device external to the RFID tag, and whereinthe first means is for storing the update value as the first calibrationvalue within the memory structure.